Fuel cell system

ABSTRACT

A fuel cell system comprises: a DBFC for generating a power by receiving a fuel; a PEMFC for generating a power by receiving hydrogen, a byproduct generated at an anode of the DBFC after a reaction, as a fuel; a supplementary power partially charged by a power generated at the DBFC and the PEMFC and discharging the charged power; a load sensing unit for sensing a load connected to the DBFC, the PEMFC, and the supplementary power; and a control unit for controlling a power of the DBFC, the PEMFC, and the supplementary power according to a load sensed by the load sensing unit and thereby selectively supplying to the load. According to this, hydrogen generated at the DBFC is recycled, and a load amount is sensed thus to stably correspond to a load variation, thereby maximizing a fuel usage efficiency and stably driving the system.

TECHNICAL FIELD

The present invention relates to a fuel cell system, and moreparticularly, to a fuel cell system capable of maximizing a fuel usageefficiency by stably corresponding to a load variation, and acontrolling method thereof.

BACKGROUND ART

A fuel cell is being developed as a replacement of a fossil fuel that isnot eco-friendly. Differently from a general secondary cell, the fuelcell is for directly converting an energy difference between before andafter a reaction generated as hydrogen and oxygen are electrochemicallyreacted into electric energy without a fuel combustion (oxidationreaction) by supplying a fuel (hydrogen or hydrocarbon) to an anode andsupplying oxygen to a cathode.

The fuel cell is being developed to be applied variously as a domesticfuel cell for supplying electricity to a home, a fuel cell used in anelectricity car, a fuel cell used in a mobile terminal or a notebookcomputer, a fuel cell movable at home and supplying electricity, etc.

Especially, a fuel cell for operating home electronics or other electricdevices by being moved at home or at an outdoors has to be minimized inorder to be conveniently portable, and has to maximize a fuel usageefficiency under a state that the size thereof is limited.

The fuel cell includes a phosphoric acid fuel cell, an alkaline fuelcell, a proton exchange membrane fuel cell(PEMFC), a molten carbonatefuel cell, a solid oxide fuel cell, a direct methanol fuel cell, adirect borohydride fuel cell(DBFC), etc.

As a fuel of the DBFC, KBH₄, NaBH₄, etc. for decomposing hydrogen areused. In case that NaBH₄ is used as a fuel, electrolyte aqueoussolution, NaOH or KOH, etc. is added. In case that NaBH₄ is used as afuel and NaOH is used as electrolyte aqueous solution, a followingreaction is performed in the anode.2H₂O+NaBH₄→NaBO₂+4H₂

As shown in the above formula, H₂ is generated during a power generatingprocess in the fuel cell. The H₂ having a great explosive characteristichas to be safely processed. In a fuel efficiency aspect, it ispreferable to re-use said H₂ by recycling.

Since an amount of a load used at home or in a factory, etc. isvariable, it is preferable to control a power generation amountaccording to the load amount.

However, controlling a power generation amount by controlling a fuelamount or a catalyst amount causes a response time to be late, therebymaking a fast reaction impossible. According to this, it is impossibleto react to an instantaneous drastic increase of a power thereby not tobe able to supply a power stably. Also, in case that a great load isdrastically applied in a no-load state, an overshooting phenomenon thata voltage is instantaneously lowered greatly at a unit cell and thelowered voltage is not recovered well is generated more distinctly thana case that a load is gradually increased. Especially, in case that avoltage deviation exists between unit cells at a stack where an anodeand a cathode are stacked, a unit cell having a low voltage is greatlydamaged by a very low voltage.

DISCLOSURE

Therefore, it is an object of the present invention to provide a fuelcell system capable of maximizing a fuel usage efficiency and beingstably operated by stably corresponding to a load variation by sensing aload amount by recycling H₂ generated at a DBFC, and a controllingmethod thereof.

To achieve these objects, there is provided a fuel cell systemcomprising: a DBFC for generating a power by receiving a fuel; a PEMFCfor generating a power by receiving hydrogen, a byproduct generated atan anode of the DBFC after a reaction, as a fuel; a supplementary powerpartially charged by a power generated at the DBFC and the PEMFC anddischarging the charged power; a load sensing unit for sensing a loadconnected to the DBFC, the PEMFC, and the supplementary power; and acontrol unit for controlling a power of the DBFC, the PEMFC, and thesupplementary power according to a load sensed by the load sensing unitand thereby selectively supplying to the load.

To achieve these objects, there is also provided a method forcontrolling a fuel cell system comprising: a first step of generating apower by driving a DBFC using a power of a supplementary power at thetime of an initial system driving, and supplying hydrogen, a byproductafter a reaction to a PEMFC; a second step of geneo a PEMFC; a secondstep of geneMFC using hydrogen supplied in the first step; a third stepof measuring a consumption power of the load that consumes a powergenerated at the DBFC and the PEMFC; and a fourth step of dischargingthe supplementary power when the consumption power of the load measuredin the third step is more than a sum of the power generated at the DBFCand the PEMFC, and charging the supplementary power when the measuredconsumption power of the load is less than the sum of the powergenerated at the DBFC and the PEMFC.

DESCRIPTION OF DRAWINGS

FIG. 1 is a construction view of a fuel cell system according to anembodiment of the present invention;

FIG. 2 is a construction view showing a structure of a DBFC;

FIG. 3 is a construction view showing a structure of a PEMFC;

FIG. 4 is a flow chart showing a signal transmission order of the fuelcell system according to a first embodiment of the present invention;

FIG. 5 is a construction view showing a second embodiment of the presentinvention;

FIG. 6 is a flow chart of a controlling method of the fuel cell systemfor determining a driving method according to a first embodiment of thepresent invention; and

FIG. 7 is a flow chart of a controlling method of the fuel cell systemaccording to a second embodiment of the present invention.

BEST MODE

Hereinafter, a fuel cell system according to the present invention willbe explained as follows with reference to the attached drawings.

FIG. 1 is a construction view of a fuel cell system according to anembodiment of the present invention.

As shown, the fuel cell system according to the present inventioncomprises: a DBFC 100 for generating a power by receiving a fuel; aPEMFC 300 for generating a power by receiving hydrogen, a byproduct atan anode of the DBFC 100 after a reaction, as a fuel; a supplementarypower 500 partially charged by a power generated at the DBFC 100 and thePEMFC 300 and discharging the charged power; a load sensing unit (notshown) for sensing a load connected to the DBFC 100, the PEMFC 300, andthe supplementary power 500; and a control unit (not shown) forcontrolling a power of the DBFC 100, the PEMFC 300, and thesupplementary power 500 according to a load sensed by the load sensingunit and thereby selectively supplying to the load.

FIG. 2 is a construction view showing a structure of the DBFC.

As shown, the DBFC 100 includes: a fuel cell stack 110 where an anode111 and a cathode 112 are arranged under a state that an electrolytemembrane (not shown) is disposed therebetween; a fuel tank 121 forstoring a fuel; a fuel pump 122 for pumping a fuel stored in the fueltank 121 to the anode 111 of the fuel cell stack 110; an air supply unit130 connected to the anode 112 of the fuel cell stack 110 by an airsupply line, for supplying oxygen, etc. to the cathode 112; a gas/liquidseparator 123 for separating a fuel, air, and a byproduct remaining atthe fuel cell stack 112 after a reaction into gas and liquid; and ahydrogen supply unit 150 for supplying hydrogen separated by thegas/liquid separator 123 to the PEMFC.

The air supply unit 130 includes: an air compressor 131 for supplyingair in the atmosphere to the cathode 112 of the fuel cell stack 110; anair filter 132 for filtering air supplied to the fuel cell stack 110; ahumidifier 133 for humidifying air supplied to the fuel cell stack 110;and a water tank 134 for supplying moisture to the humidifier 133.

The hydrogen supply unit 150 preferably controls hydrogen supplied tothe PEMFC to be supplied with a certain amount.

As a fuel supplied to the DBFC 100, one of NaBH₄, KBH₄, LiAlH₄, KH, NaH,etc., and one of electrolyte aqueous solution such as NaOH, KOH, etc.can be used.

FIG. 3 is a construction view showing a structure of the PEMFC.

As shown, the PEMFC 300 includes: a fuel cell stack 310 where an anode311 that receives hydrogen generated at the DBFC 100 and a cathode 312are arranged under a state that an electrolyte membrane (not shown) isdisposed therebetween; an air compressor 331 for supplying air in theatmosphere to the cathode 312 of the fuel cell stack 310; an air filter332 for filtering air supplied to the fuel cell stack 310; a heatexchanger 333 for humidifying and heating air supplied to the fuel cellstack 310; and an evaporator 323 for evaporating a material remaining atthe cathode 312 after a reaction.

As the supplementary power 500, any unit that can control a charging anda discharging is possible. Also, a battery or a capacitor can be used asthe supplementary power. In case that the supplementary power is formedof an electric device, a fast reaction to a load variation can beperformed by a control unit 400 since a time constant of the electricdevice is less than that of the DBFC or PEMFC that generates a power byusing a general chemical reaction.

The supplementary power 500 is preferably connected to an external powerof the fuel cell system thus to be charged by the external power at thetime of being completely discharged.

The control unit 400 includes: boosters 410, 420, and 430 respectivelyconnected to the DBFC 100, the PEMFC 300, and the supplementary power500 in series, for boosting a voltage; and an inverter 440 connected tothe boosters 410, 420, and 430, for converting a direct current into analternating current.

The boosters 410, 420, and 430 preferably boost a voltage of the DBFC100, the PEMFC 300, and the supplementary power 500 into 350V.

The inverter 440 preferably converts a voltage of 330V, a directcurrent, into 220V, an alternating current, that is commonly used athome.

It is preferable that the control unit 400 further includes a buckconverter 450 connected to the boosters 410, 420, and 430 for convertinga direct current into a direct current.

The buck converter 450 is a kind of a switched-mode power supply, and isa device for converting a DC input voltage into a voltage of a squarewave by using a semi-conductor device such as a MOSFET for power, etc.as a switch, and then obtaining a DC output voltage controlled by afilter.

FIG. 4 is a flow chart showing a signal transmission order of the fuelcell system according to a first embodiment of the present invention.

A size of a load 600 connected to the DBFC 100, the PEMFC 300, and thesupplementary power 500 is real-time measured by a load sensing unit 200thus to be transmitted to the control unit 400. The control unit 400determines a driving method of the DBFC 100, the PEMFC 300, and thesupplementary power 500 according to an inputted algorithm. By thedetermined driving method, a power is supplied to the load 600.

FIG. 5 is a construction view showing a second embodiment of the presentinvention.

As shown, a second sensing unit 250 is connected to each unit cell 720of a fuel cell stack 710 where an anode 711 and a cathode 712 arestacked, and measures a voltage of the unit cell 720.

A second control unit 460 is connected to the second sensing unit 250thus to receive a signal. If a voltage of the unit cell is less than apreset voltage, the second control unit 460 complements a voltage byusing the supplementary power 500.

A controlling method of the fuel cell system according to the firstembodiment of the present invention will be explained as follows.

The method for controlling a fuel cell system comprises: a first step ofgenerating a power by driving a DBFC using a power of a supplementarypower at the time of an initial system driving, and supplying hydrogen,a byproduct after a reaction to a PEMFC; a second step of generating apower by driving the PEMFC using hydrogen supplied in the first step; athird step of measuring a consumption power of the load that consumes apower generated at the DBFC and the PEMFC; and a fourth step ofdischarging the supplementary power when the consumption power of theload measured in the third step is more than a sum of the powergenerated at the DBFC and the PEMFC, and charging the supplementarypower when the measured consumption power of the load is less than thesum of the power generated at the DBFC and the PEMFC.

The first step is composed of: a power generating step for generating apower at the DBFC by using the supplementary power; and a hydrogensupplying step for supplying hydrogen generated at the DBFC after areaction to the PEMFC.

Once the fuel cell system is operated by a user, the DBFC is operatedthus to supply NaBH₄, etc. and electrolyte aqueous solution such asNaOH, etc. to an anode, and to supply oxygen-including air to a cathode.At this time, a pump arranged at the anode or a compressor arranged atthe cathode is operated by using a power of the supplementary power.According to this, a power is generated, and a byproduct such ashydrogen, etc. is generated at the DBFC after a reaction. The generatedhydrogen is supplied to the PEMFC as a fuel.

The second step is for generating a power at the PEMFC by supplyinghydrogen to an anode and by supplying oxygen-including air to a cathode.As a power for operating components of the PEMFC (for example, a pump ora compressor), a power generated at the DBFC and the PEMFC is partiallyused.

The third step is for detecting a load amount by a load sensing unit andthereby transmitting the signal to a control unit.

The fourth step is for determining a driving method of the DBFC, thePEMFC, and the supplementary power, which will be explained in moredetail.

FIG. 6 is a flow chart of the controlling method of the fuel cell systemfor determining a driving method of the DBFC, the PEMFC, and thesupplementary power according to a first embodiment of the presentinvention.

As shown, A denotes a power amount generated at the DBFC, B denotes apower amount generated at the PEMFC, and C denotes a measured load size.When the measure load size C is more than or the same as a sum of thepower generated at the DBFC and the PEMFC (A+B), the supplementary poweris discharged. Herein, an amount discharged from the supplementary poweris a difference between the measured load size and the sum of the powergenerated at the DBFC and the PEMFC (C−(A+B)). If the measured load sizeC is less than the sum of the power generated at the DBFC and the PEMFC,the supplementary power is charged. Herein, a charged amount of thesupplementary power is a value obtained by deducting the measured loadsize from the sum of the power generated at the DBFC and the PEMFC((A+B)−C).

The charged supplementary power is used at the time of a drastic loadincrease, or is used to drive a component of the DBFC at the time of theinitial system driving, or is used to recollect a fuel remaining at onecomponent of the DBFC to another preset component of the DBFC.

The controlling method of the fuel cell system further comprises a fifthstep of recollecting a fuel remaining at one component of the DBFC toanother preset component of the DBFC by using a power of the PEMFC whenthe system is stopped.

NaOH, etc. used as electrolyte aqueous solution at the DFBC has a strongcorrosion characteristic thus to corrode a component connection line ofthe DBFC. Therefore, it is necessary to recollect the NaOH, etc. to apreset component. When the system is stopped by the user, the DBFC isimmediately stopped but a power can be generated at the PEMFC by usinghydrogen that is not consumed yet. Therefore, it is necessary torecollect a fuel remaining at the DBFC line by using a power generatedat the PEMFC, and by using the supplementary power when the powergenerated at the PEMFC is not sufficient.

FIG. 7 is a flow chart of the controlling method of the fuel cell systemaccording to a second embodiment of the present invention.

The controlling method of the fuel cell system comprises: a first stepof generating a power by driving a DBFC using a power of a supplementarypower at the time of an initial system driving, and supplying hydrogen,a byproduct after a reaction to a PEMFC; a second step of generating apower by driving the PEMFC using hydrogen supplied in the first step; athird step of measuring a voltage of each unit cell of the DBFC and thePEMFC; and a fourth step of discharging the supplementary power for acertain time when the voltage measured in the third step is less than orthe same as a preset voltage, and charging the supplementary power whenthe measured voltage is more than the preset voltage.

When a voltage of the unit cell is drastically lowered due to anovershooting phenomenon generated as a load is drastically increased,the voltage of the unit cell is real-time checked. If the checkedvoltage is less than or the same as a preset voltage value D1, thesupplementary power is discharged thus to complement a voltage. On thecontrary, if the voltage of the unit cell is more than D2 that is morethan D1 as the temperature of the unit cell is increased after a certaintime T lapses, it is judged as that the unit cell has recovered itsfunction. According to this, the discharging of the supplementary poweris stopped.

INDUSTRIAL APPLICABILITY

As aforementioned, in the fuel cell system of the present invention,hydrogen generated at the DBFC is recycled thus to restrain a dischargeof the explosive hydrogen, and the supplementary power is selectivelydriven by detecting a load amount. According to this, it is possible tocorrespond to an instantaneous variation of a load and thereby to stablyoperate the system. Also, it is possible to correspond to a drastic loadvariation by using the supplementary power even under a state that acapacity of the DBFC and the PEMFC is less. According to this, a cost isreduced, and a compact system can be constructed. Additionally, adrastically decreased voltage of the unit cell due to a drastic loadvariation can be complemented by using the supplementary power, therebygradually increasing a load amount.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover modifications and variationsof this invention provided they come within the scope of the appendedclaims and their equivalents.

1. A fuel cell system comprising: a DBFC for generating a power byreceiving a fuel; a PEMFC for generating a power by receiving hydrogen,a byproduct generated at an anode of the DBFC after a reaction, as afuel; a supplementary power partially charged by a power generated atthe DBFC and the PEMFC and discharging the charged power; a load sensingunit for sensing a load connected to the DBFC, the PEMFC, and thesupplementary power; and a control unit for controlling a power of theDBFC, the PEMFC, and the supplementary power according to a load sensedby the load sensing unit, and thereby selectively supplying to the load.2. The system of claim 1, wherein the DBFC includes: a fuel cell stackwhere an anode and a cathode are arranged under a state that anelectrolyte membrane is disposed therebetween; a fuel tank for storing afuel; a fuel pump for pumping a fuel stored in the fuel tank to theanode of the fuel cell stack; an air supply unit connected to the anodeof the fuel cell stack by an air supply line, for supplying oxygen tothe cathode; a gas/liquid separator for separating a fuel, air, and abyproduct remaining at the fuel cell stack after a reaction into gas andliquid; and a hydrogen supply unit for supplying hydrogen separated bythe gas/liquid separator to the PEMFC.
 3. The system of claim 2, whereinthe air supply unit includes: an air compressor for supplying air in theatmosphere to the cathode of the fuel cell stack; an air filter forfiltering air supplied to the fuel cell stack; a humidifier forhumidifying air supplied to the fuel cell stack; and a water tank forsupplying moisture to the humidifier.
 4. The system of claim 1, whereinthe PEMFC includes: a fuel cell stack where an anode that receiveshydrogen generated at the DBFC and a cathode are arranged under a statethat an electrolyte membrane is disposed therebetween; an air compressorfor supplying air in the atmosphere to the cathode of the fuel cellstack; an air filter for filtering air supplied to the fuel cell stack;a heat exchanger for humidifying and heating air supplied to the fuelcell stack; and an evaporator for evaporating a material remaining atthe cathode after a reaction.
 5. The system of claim 1, wherein the loadsensing unit further includes a second sensing unit connected to a unitcell of the fuel cell stack that the anode and the cathode are stacked,for sensing a voltage of the unit cell.
 6. The system of claim 5,wherein the control unit further includes a second control unitconnected to the second sensing unit thus to receive a signal thereof,for complementing a voltage using the supplementary power when a voltageof the unit cell is less than a preset voltage.
 7. The system of claim1, wherein the supplementary power is a capacitor.
 8. The system ofclaim 1, wherein the supplementary power is a battery.
 9. The system ofclaim 1, wherein the supplementary power is connected to an externalpower of the fuel cell system thus to be charged by the external powerat the time of being completely discharged.
 10. The system of claim 1,wherein a fuel supplied to the DBFC is NaBH₄.
 11. The system of claim 1,wherein the control unit includes: boosters respectively connected tothe DBFC, the PEMFC, and the supplementary power in series, for boostinga voltage; and an inverter connected to the boosters, for converting adirect current into an alternating current.
 12. The system of claim 11,wherein the control unit further includes a buck converter connected tothe boosters for converting a direct current into a direct current. 13.A controlling method of a fuel cell system comprising: a first step ofgenerating a power by driving a DBFC using a power of a supplementarypower at the time of an initial system driving, and supplying hydrogen,a byproduct after a reaction to a PEMFC; a second step of generating apower by driving the PEMFC using hydrogen supplied in the first step; athird step of measuring a consumption power of a load that consumes apower generated at the DBFC and the PEMFC; and a fourth step ofdischarging the supplementary power when the consumption power of theload measured in the third step is more than a sum of the powergenerated at the DBFC and the PEMFC, and charging the supplementarypower when the measured consumption power of the load is less than thesum of the power generated at the DBFC and the PEMFC.
 14. The method ofclaim 13, wherein the first step includes: a power generating step forgenerating a power at the DBFC by using the supplementary power; and ahydrogen supplying step for supplying hydrogen generated at the DBFCafter a reaction to the PEMFC.
 15. The method of claim 13 furthercomprising a fifth step of recollecting a fuel remaining at onecomponent of the DBFC to another preset component of the DBFC by using apower of the supplementary power when the system is stopped.
 16. Themethod of claim 13 further comprising a fifth step of recollecting afuel remaining at one component of the DBFC to another preset componentof the DBFC by using a power of the PEMFC when the system is stopped.17. A controlling method of a fuel cell system comprising: a first stepof generating a power by driving a DBFC using a power of a supplementarypower at the time of an initial system driving, and supplying hydrogen,a byproduct after a reaction to a PEMFC; a second step of generating apower by driving the PEMFC using hydrogen supplied in the first step; athird step of measuring a voltage of each unit cell of the DBFC and thePEMFC; and a fourth step of discharging the supplementary power for acertain time when the voltage measured in the third step is less than orthe same as a preset voltage, and charging the supplementary power whenthe measured voltage is more than the preset voltage.